Bis(N,N-diethyl-4-methyl-4-piperazine-1-carboxamide) tetrakis(isothiocyanato-κN)cobalt(II), a model compound for the blue color developed in the Scott test

The structure of bis(N,N-diethyl-4-methyl-4-piperazine-1-carboxamide) tetrakis(isothiocyanto)cobalt(II) [or bis(carbamazide) tetrakis(isothiocyanato)cobalt(II)] is reported. This complex represents an example of the compound found in the colormetric response in Scott’s test.


Chemical context
In forensics and law enforcement, the Scott test, and modifications to that test (Scott, 1973;Fansello & Higgins, 1986;Tsujikawa et al., 2017), provide identification of tertiary amines from opioids present in a sample. However, there are few reports on the nature of the coloration that is observed during this test, which can vary from powder blue to royal blue, purple, blue-green, or green depending on the identity of the tertiary amine being tested. Oguri and co-workers found that the blue precipitates from cocaine hydrochloride have a 1:2 cobalt:cocaine stoichiometry (Oguri et al., 1995), and IR spectra show the blue precipitates contain one or more thiocyanate units (Morris, 2007). However, the strong blue color is consistent with a tetrahedral Co II species, rather than the octahedral structure postulated by Oguri and co-workers.
As part of our on-going research into detection of functional groups using Paper Analytical Devices (PADs, Weaver et al., 2013;idPADs Lockwood et al., 2020), we sought to understand why tertiary amines give blue precipitates of so many colors in the presence of the Scott reagent. The citrate salt (diethylcarbamazinium citrate; CAS#1642-54-2) of a suitable tertiary amine (diethylcarbamazine; CAS#90-89-1) was selected as a representative tertiary amine. The title compound was prepared by extraction into a CH 2 Cl 2 solution from a dried, stoichiometric mixture (1:2) of K 2 [Co(NCS) 4 ] and diethylcarbamazinium citrate that yielded the blue crystals used in this study. The tetrahedral ion [Co (NCS) 4 ] 2À can also be readily formed by disproportionation of the reagent used for the Scott test [the neutral compound Co(SCN) 2 ] in the presence of a suitable amine, as demonstrated in the synthesis for the isothiopendylium tetrakis(isothiocyanato)cobalt(II) complex (refcode: QUXKOK, Arunkashi et al., 2010), which, like our structure, is an ion pair between two protonated amines and [Co (NCS) Eisman et al. (1992). However, there is still no crystal structure for the Scott test product with cocaine, and only three examples are available for protonated tertiary amine ion pairs with the [Co(NCS) 4 ] 2À dianion.
The Scott test is a three-step sequence of reactions: (1) addition of 2% cobalt thiocyanate in water; (2) addition of 1.2 M HCl solution; (3) addition of chloroform. We ascribe the initial blue precipitate in the Scott test to the formation of the ion pair (amineH) 2 [Co(NCS) 4 ]. Formation of the ion pair should be a reversible reaction, so when concentrated HCl is added in the second step and it protonates the thiocyanate ions (pK for HNCS is 1.1), the tetrahedral cobalt anion falls apart and the blue color vanishes. When chloroform is added in the final step of the Scott test, the hydrophobic ion pair reforms in the organic solvent, turning it blue.

Structural commentary
The complex crystallizes with two protonated diethylcarbamazine cations and one tetrakis(isothiocyanato)cobalt(II) dianion in the asymmetric unit (Fig. 1). The isothiocyanate ligands are bound to the cobalt through their nitrogen atoms, leaving the more bulky and hydrophobic sulfur atoms exposed to the solvent. Protonation of the carbamazines was confirmed by the presence of electron density on the methyl-piperazine nitrogen atoms N6 and N9. The geometry of the carbamazide molecules is unexceptional. The Co II center adopts a near ideal tetrahedral geometry ( 4 = 0.97; Yang et al., 2007; Table 1) that is located in a general position within the asymmetric unit. In contrast, the cobalt center in the parent compound K 2 [Co(NCS) 4 ]Á3H 2 O is located on a twofold screw-axis (space group P2 1 2 1 2; Drew & Othman, 1975). The 4 metric for the parent compound is 0.94 with the largest N-Co-N angle = 114.1 (3) [in contrast to 112.10 (7) reported here]. Although this small change should be considered carefully because the Scott test result is a solution phase analysis and here the solid-state structures are compared, it could indicate that the colorimetric response is a change in the tetrahedral ligand field about Co.

Synthesis and crystallization
K 2 [Co(NCS) 4 ] was prepared by the metathesis of Co(NO 3 ) 2 (3.00 g, 16.4 mmol) and K(SCN) (3.88 g, 39.9 mmol) in 20 mL of water and allowed to dry. Dark-blue crystals were harvested for subsequent reactions; note: upon dissolution in water the solution is pink. K 2 [Co(NCS) 4 ] and diethylcarbamazide citrate were mixed in a stoichiometric (1:2) ratio in water and allowed to dry. CHCl 3 or CH 2 Cl 2 was added to extract the blue complex. Crystals were grown from the CH 2 Cl 2 extract by vapor diffusion of hexane at 277 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms bonded to tertiary amine nitrogen atoms (N6, N9) were refined freely. All other hydrogen atoms were included in geometrically calculated positions with C-H bond distances constrained to 0.98 Å for aromatic and methylene and 0.99 Å for methyl hydrogen atoms with U iso (H) = 1.5U eq (C) for methyl and 1.2U eq (C) for aromatic and methylene H atoms.     program(s) used to solve structure: SHELXT2014/5 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2020); software used to prepare material for publication: publCIF (Westrip, 2010).

Bis(N,N-diethyl-4-methyl-4-piperazine-1-carboxamide) tetrakis(isothiocyanto-κN)cobalt(II)
Crystal data (C 10  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.